1. Field of The Invention
The present invention is directed to imaging systems, and more particularly to thermal or infrared imaging systems.
2. Related Art
Thermal or infrared imaging systems have increased in importance in recent years. FIG. 1 is a cross-sectional view of a conventional infrared imaging system 101. The infrared imaging system 101 contains an IR detector 108 (also called focal plane array 108).
In order to keep the focal plane array 108 at a cryogenically cool temperature appropriate for thermal energy sensing (typically 77.degree. kelvin or cooler), the focal plane array 108 is integrated into a dewar assembly 102. The dewar 102 is essentially a small insulated container. A cold finger 110 contacts the focal plane array 108 to keep the focal plane array 108 cold. The cold finger 110 is cryogenically cooled from a gas bottle or a cryoengine.
The dewar 102 includes a dewar window 104. The dewar window 104 allows thermal energy into the dewar 102 but also serves as a vacuum seal for the dewar 102.
The infrared imaging system 101 also includes a cold shield 106 (also called a cold stop). The cold shield 106 is uniformly cooled and hence emits little or no thermal energy. The cold shield 106 serves the purpose of limiting the solid angle viewed by the focal plane array 108.
The need for highly efficient cold shielding of the focal plane array 108 is inherent in any high performance infrared imager 101. For conventional scanning infrared focal plane array sensors, such high efficiency cold shielding is normally achieved by making the cold shield aperture (within the dewar 102) the stop of the optical system 101. By doing this, the focal plane array 108 "sees" (in a thermodynamic sense) only the wanted cones of image forming light surrounded by a cold background represented by the cold shield.
In some high performance infrared imagers, however, the focal plane array 108 is perforce located far away from the optical stop of the optical system. Thus, an external stop 112 is used. The dewar 102 in which the focal plane array 108 is located is limited in length by cool-down time and other system constraints. Because of the location of the cold shield aperture, it is impossible for the cold shield itself to act as a stop of the optical system. Thus, an external stop is used. This is further described below with reference to FIGS. 2A and 2B.
FIG. 2A illustrates a cross-sectional view of a conventional infrared imaging system 101A. As shown in FIG. 2A, conventionally the infrared imaging system 101A is formed in an optical barrel 202. Unlike the cold shield 106, the optical barrel 202 is not cooled. Therefore, the optical barrel 202 emits thermal energy. For readability purposes, only half of the optical barrel 202 is shown.
FIG. 2A illustrates a desired light bundle 204 (one of the many which together image onto a focal plane array 108). Light bundle 204 is limited by the external stop 112 and focused on the focal plane array 108. In this patent document, the terms "light", "ray", and the like refer to the thermal (or infrared) energy contained in such lights or rays.
Also shown in FIG. 2A is undesired light 206 which is typical of that emitted by the inside of the optical barrel 202. The IR focal plane array 108 receives both the desired light bundle 204 and the undesired light 206. The reception of the undesired light 206 degrades the sensing performance of the infrared imaging system 101.
FIG. 2B illustrates a cross-sectional view of a modified conventional infrared imaging system 101B. The modified infrared imaging system 101B includes cold shield extensions 208. The cold shield extensions 208 are advantageous because they block the undesired light 206 from contacting the focal plane array 108. In other words, the cold shield 106 itself acts as a stop of the optical system. Again, only half the optical barrel 202 is shown for readability purposes.
Note, however, that the cold shield extensions 208 are undesirable because they also block the desired light bundle 204 from contacting the IR focal plane array 108. Therefore, the cold shield 106 cannot be modified to block the undesired light 206 because such modifications to the cold shield 106 would also result in a blockage of the desired light bundle 204. In other words, the cold shield 106 cannot act as a stop of the optical system.
Therefore, it is necessary to develop a means by which the focal plane array 108 sees a cold background even though the cold shield aperture does not define the external stop 112.